The mechanism responsible for the colonization of the respiratory tracts of patients with cystic fibrosis by mucoid strains of Pseudomonas aeruginosa is being studied. These studies suggest that lytic bacteriophage and several aeruginocins exert selctive pressure on the population of Pseudomonas aeruginosa in the cystic fibrosis airways, due to the relative resistance of the mucoid strains to phage or aeruginocin-induced bacterial killing.